Nearly all prior art sewage purification systems require that sooner or later the system be closed down to allow removal of sludge that has not been fully treated and has accumulated in the processing vessels. Large municipal treatment plants have the equipment and personnel to carry out this work. However, small-scale systems intended for the use of a single house or housing blocks are better served by arrangements that almost completely dispose of organic solids and so do not require such servicing.
As is known, environmental regulations are becoming increasingly stringent, and the legal disposal of wastes, whether as solids, liquids or even as gases, is becoming increasingly difficult and expensive.
It is known that, besides carbon dioxide and minor quantities of other gases, methane is produced during the decomposition of sludge material. Typically, 0.35 m.sup.3 methane becomes available per kilogram of sludge decomposed. In the U.S., methane produced by cattle and expelled through their digestive systems is considered such an environmental nuisance that Congress has funded research to see what, if anything, can be done about this. Interestingly, methane has a higher heat value than any other common fuel gas, except for hydrogen. However, in almost all prior art systems, generated methane is discharged into the atmosphere, causing an unpleasant odor. In some localities municipal regulations prohibit discharge of such gases.
Methods and apparatus for treating domestic effluents are disclosed in U.S. Pat. No. 4,172,034 (Carlsson, et al); U.S. Pat. No. 4,812,237 (Cawley); U.S. Pat. No. 5,114,586 (Humphrey) and U.S. Pat. No. 5,342,523 (Kuwashima).
Carlsson describes an apparatus which operates on an easy-flowing slurry, having a dry solids content of between 1-15%, preferably 5-10%. Such a dilute slurry unnecessarily extends processing time to achieve aerobic degradation in a reaction vessel with aeration; however, the Carlsson apparatus has the advantage of being compact.
Humphrey discloses a complex sanitation system provided with many vessels, five of which have multiple air entry orifices. The resulting high air consumption necessitates the installation of a large air blower or compressor, leading to high running costs and a noise suppression problem. Another difficulty encountered in the Humphrey system is finding space in a residential building for all the described system components.
Cawley describes and claims a process for purifying and recycling household waste waters, comprising the steps of (a) collecting a first wastewater stream from household kitchen sources; (b) anaerobically digesting said first wastewater stream in a first septic tank; (c) collecting a second wastewater stream from household laundry and bathing sources; (d) combining water from steps (b), (c) and (h); (e) anaerobically digesting water from step (d) in a second septic tank; (f) pumping water from step (e) over a biological sand filter under aerobic conditions; (g) pumping biologically filtered water from step (f) through an ultra-filter, thereby separating the biologically filtered water into a retentate stream and a permeate stream; (h) returning said retentate stream to step (d); (i) disinfecting said permeate stream; (j) returning a first portion of said disinfected permeate stream to household laundry and bathing facilities; (k) separating a second portion of said disinfected permeate stream into a low salt portion and a high salt portion; (l) returning said low salt portion to a household kitchen; and (m) disposing of said high salt portion.
Kuwashima proposes a pair of separator tanks which are used alternately for separating floating or sedimenting material; the organic material is transferred for aerobic decomposition to a third tank. The device lacks means for breaking up large solids into small particles for efficient decomposition.